1. Field of the Invention
The present invention relates to an optical fiber applicable to an optical fiber amplifier and a process of producing the optical fiber.
2. Description of the Related Art
An optical amplifier capable of directly amplifying light signals without conversion into electric signals is advantageous in that it can be easily enlarged in capacity because of a substantially bit rate free function thereof and that it can collectively amplify multiple channels, and from this standpoint, it is being extensively studied as one of key devices of future optical communication systems by various research organizations.
There is known one form of an optical amplifier using a single mode optical fiber including a core doped with a rare earth element such as Er, Nd, or Yb (hereinafter, referred to as "a doped fiber"), wherein signal light to be amplified is transmitted to the doped fiber and at the same time pumping light is introduced into the doped fiber in the direction identical or reversed to that of the signal light.
The optical amplifier using the doped fiber, which is called an optical fiber amplifier, has excellent features of eliminating a polarization dependency of a gain, lowering a noise, reducing a loss in coupling with an optical transmission path, and the like. In practical use of the optical fiber amplifier of this type, it is required to make wider a wavelength band width of signal light in which the signal light can be amplified at a specific gain (hereinafter, referred to simply as "a wavelength band width") and to make higher a conversion efficiency of pumping light into signal light.
As for light having a wavelength within a range of 0.8 to 1.6 .mu.m, there have been established a technique of producing an optical fiber using a quartz glass suitable for long-distance transmission, and a technique of putting the optical fiber into practice. An optical fiber is obtained by drawing a preform in the shape of a thick rod. The preform is required to have a composition gradient in a cross-sectional direction thereof which is accurately set as designed.
A standard process of preparing a preform has been known, in which a glass composition chemically converted from reactive gases is deposited on an inner surface of a quartz reaction tube by a MCVD (Metal Chemical Vapor Deposition) process or the like. In the MCVD process, suitable reactive gases such as SiCl.sub.4 and O.sub.2 are introduced in the quartz reaction tube, and the quartz reaction tube is heated at a temperature suitable for reaction of the gases. A heating zone is moved in the longitudinal direction of the quartz reaction tube, to deposit a new glass layer on an inner wall surface of the quartz reaction tube. A plurality of layers, for example, 20-30 layers are repeatedly deposited. A composition gradient in the cross-sectional direction of an optical fiber produced from the preform can be controlled by independently adjusting compositions of the layers of the preform. After the layers are fully deposited, the quartz reaction tube is collapsed by heating, to be thus formed into a rod-shaped preform. The preform is then drawn to produce an optical fiber.
In the MCVD process, a reactive material vaporized at a room temperature is generally used. For example, SiCl.sub.4 is used for forming SiO.sub.2 which is a main component of an optical fiber, and GeCl.sub.4 is used for forming GeO.sub.2 which is an element for adjustment of a refractive index. Incidentally, for production of a doped fiber, a suitable reactive material containing a rare earth element sufficiently evaporated at a room temperature cannot be obtained, differently from SiCl.sub.4 and GeCl.sub.4, and consequently a rare earth element cannot be doped in the doped fiber at a practically sufficient concentration only by the MCVD process. For this reason, a rare earth element has been doped in a doped fiber at a practically sufficient concentration in the following manner.
A known process of preparing a preform suitable for production of a doped fiber includes a step (1) of depositing a soot-like core glass on an inner surface of a quartz reaction tube, a step (2) of allowing the soot-like core glass impregnated with a solution containing a rare earth element compound as a solute, and a step (3) of drying the solution and collapsing the quartz reaction tube. On the other hand, a technique of widening a wavelength band width of an optical fiber amplifier using a doped fiber has been proposed, in which a core is impregnated with Al.sub.2 O.sub.3 as well as a rare earth element.
For example, Japanese Patent Laid-open No. Hei 5-119222 discloses a double core structure including an aluminum/silica based glass (Er--Al--SiO.sub.2) doped with erbium (Er) and aluminum (Al), which is provided at a center portion of a core; and a germanium/silica based glass (Ge--SiO.sub.2) doped with germanium (Ge), which is provided at an outer peripheral portion of the core. In the prior art structure disclosed in Japanese Patent Laid-open No. Hei 5-119222, however, is disadvantageous in that a relative index difference .DELTA.1 of the core peripheral portion is about 2% but a relative index difference .DELTA.2 of the core center portion is about 0.7% at the utmost, with a result that there occurs a large depression of a refractive index at the core central portion.
This is due to the fact that an element doped for widening a band width, such as Al, acts to decrease a refractive index. The depression of a refractive index causes a phenomenon in which a mode field of transmission light is spread and thereby a mode field diameter is made larger. The mode field diameter thus increased is inconvenient in converting pumping light into signal light and results in degradation of a conversion efficiency of pumping light into signal light. For example, in the prior art structure disclosed in Japanese Patent Laid-open No. Hei 5-119222, a mode field diameter was about 4.8 .mu.m and a conversion efficiency of pumping light into signal light was 64%.